question

doubh avatar image

Supply AC loads instead of charging - behavior on cloudy days

I'm using

  • VenusOS 2.73 on Raspberry Pi
  • ESS
  • Multiplus II 48/3500/35
  • SmartSolar Charger MPPT 250/70 rev2
  • Pylontech US2000C batteries (7.2kWh)


On cloudy days, when there is a less energy from PV and the battery is already at "empty" (ESS#1), all the energy is used to charge the battery and AC loads (+critical loads) and supplied completely from the grid.

1642155122120.pngWhen the minimum soc level +3 is reached and ESS#1 is gone, the AC loads are supplied from PV and battery.

When the PV power is less than the AC loads (see number in the picture above), the battery will be discharged again.

Then after a few hours, the minimum soc level is reached again, ESS#1 is raised and all AC loads are supplied from grid again. And all PV power is used to recharge the battery.

This leads to continuous charging and discharging of the battery, on some days multiple cycles per day.


What I want to configure/do is, that in ESS#1 state, all the PV power is used to supply AC loads (+critical loads).

Just the excess energy should be used to charge the battery.

This would lower or even prevent the described charge-discharge cycles and it would increase the efficiency, since the energy is not stored in battery first, but is used immediately.


I'm familiar with coding, so if there is no config-way to achieve it, there may be a way coding way to achieve it.

ESS
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3 Answers
Alexandra avatar image
Alexandra answered ·

@doubh

Do you have Optimized with battery life on or off?

Usually with it on it priorities getting the battery bank to full charge and tries to keep it operating up there.

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Batterylife has no bearing on the priority of PV, ESS works the same way. Only the actual minimum SOC will vary based on the previous charging history.

Keep batteries charged will get the battery to 100% and then leave it there with no discharge.

There is no way to force ESS to divert all PV to AC loads once at a minimum SOC.

ESS is a battery-centric technology, either prioritising it to be charged, or discharging it to cover loads.

Optimized battery life is OFF

I want the exact opposite of what you described: Instead of charging the battery, the AC load (+critical load) should be prioritized above the battery.

If PV power is higher than AC loads, this excess power should be used to charge the battery.

In my posted image, I want his behavior:

  • 246W from PV Charger used to supply Critical Loads 44W and the rest 202W for AC Loads.
  • Grid would be 165W (theoretically - I know that there are conversion losses)

Example when there is more PV power, lets assume 600W:

  • 600W from PV Charger used to supply Critical Loads 44W and 367W for AC Loads
  • Grid would be 0
  • The rest 189W (600W-44W-367W) should be used to charge to the battery.


When ESS#1 is left, everything is fine and should be as it is.

You may be better off with a PV inverter on the AC end.

PV inverter on AC and has a poor efficiency when using a battery - this is not an option. A DC coupled system has many advantages.

ESS has the abilities to control the energy flow in the way I suggest. I just looks like that in mode 1+2, this feature [AC prior battery-charge] does not exist.

ESS is not designed to work this way.


ESS already works in that way, when battery soc is above minimum SOC +3:

  1. AC loads have highest prio
  2. If AC loads < PV power, battery will be charged
  3. If SOC == 100, excess energy is fed into grid


When minimum SOC level is reached, discharge is completely disabled until it is recharged up to minimum SOC +3.

And I want to change this phase:

  • use PV power for AC loads,
  • charge battery only if PV power > AC loads and
  • keep battery discharge disabled, of course.
doubh avatar image
doubh answered ·

The SOC graph looks like below, when AC loads and PV power stay in the same range. Charge followed by discharge aso...

This could be prevented when PV power is directed to AC loads.

1642177383246.png


1642177383246.png (60.5 KiB)
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d3-energy avatar image
d3-energy answered ·

The charge and discharge of the battery does not really count as a cycle for the battery within that small SOC range. If it were detrimental to the battery, ESS would not be designed that way.

You mention that coding is an option.
You could optionally run ESS in mode 2 or 3 to change the default behavior of ESS.
More info: https://www.victronenergy.com/live/ess:ess_mode_2_and_3

Venus OS Large has node red as a component that you can possibly use:
https://www.victronenergy.com/live/venus-os:large

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I made a small Pyhton script that dynamically sets DBus item /Settings/CGwacs/MaxDischargePower depending on PV power info from DBus '/Dc/Pv/Power'.

This allows using the PV power for AC loads instead of charging the battery (only when the SOC is low).

Thank you for the feedback!